Mikeaagesen

NZ,

Thanks for clearing up my confusion. So this means that only the rears are adjustable? Then again, if the rears are adjustable, you wouldnt need to ever adjust the fronts right?

z3bra

Mike,

You always want full pressure to the fronts. As you stop, you experience dive from the center of gravity effectively shifting toward the front of the vehicle, the front brakes do more of the braking load. The harder you stop, the larger their load becomes. (a 3:1 ratio is commonly expressed as the difference between the two for cars in general and before anyone pipes up with more technical data I'm just generalizing here but feel free to show more accurate data if you'd like as I would like the correct data if I'm way off.) On the other hand as the effective center of gravity moves toward the front of the car you effectively have less of the mass of the vehicle resting on the rear tires' contact patch and as such the traction is reduced on the rear tires. Because of the reduced traction it becomes increasingly easier to lock the rear brakes. Because of this, you don't want the full pressure to the rears as they will lock up and bad things happen when the rears lock on you.

Hell, let off the throttle going fast enough and even the engine braking effect can do the same thing to a lesser extent with the potential for equally disasterous results, A good example was a some video footage of a stolen Corvette being persued on the freeway by the Arizona Highway Patrol maybe 8 or so years ago, the driver of the Vette let up on the throttle way to fast with the car in gear to avoid a tractor-trailer in his way and this made the car spin almost instantly causing him to wreck the Vette. He walked away from it but it obviously didn't help him avoid getting arrested almost immediately thereafter.

The opposite effect is "squat" when you take off and the center of gravity shifts toward the rear of the vehicle. This is why rear wheel drive cars have an inherent advantage in drag racing for instance with all other things being equal. The transfer of the effective center of gravity just increases the traction on the rear wheel drive car while it decreases it on a front wheel drive car. All wheel drive is better still in that it still takes advantage of what traction is available on the front tires as well but if you have a poorly biased center differential you can defeat that advantage by inducing wheel spin in the front axle. Look at the bias valve as somewhat like the center differential in an all wheel drive system. You want to keep maxium power to the fronts in an AWD launch without causing wheelspin. Any less is not as fully efficient while too much just wastes power from spinning the wheels instead of propelling the car at the launch.

The problem with the opposite phenomenon as observed with braking as opposed to an AWD launch is that a bad AWD launch means you post a bad time and lose that race. A bad stop from the rears locking up tends to make your car spin and generally if you're stopping that hard you're still going to be going fast enough to break things. Breaking things on cars is expensive and nobody likes to break things. (Well maybe guys in a demolition derby do, but they probably got the car for free anyway and aren't planning on keeping it usually.)

The bias valve is what limits the pressure to the rear brakes. In other words it biases the pressure so that the front brakes receive the full pressure and the rears receive part of the pressure or more correctly they receive full pressure until that amount of pressure reaches a set point after which they will not receive any more pressure. The bias valve is simply a pressure limiting valve.

While adjustable bias valves certainly exist, it's not something you want to fool with with unless you're up to track testing it to achieve as much braking force in the back as you can without lockup under all forseeable conditions. Since the 5/33 is fairly well established as being predictable it's a relatively safe bet and a quick and easy upgrade it is used pretty often. Of course even running the 5/33 you can be more prone to locking the rears on a wet road with a stop that isn't straight on. Now under ideal and track conditions we know better than to brake hard on a curve, especially on a wet roadway but for street driving sometimes we don't always have that luxury. The bottom line is, it's safer to err on the side of caution in rear brake bias and have too little going to the rears at the cost of decreased overall stopping power in favor of increased predictability and a lower likelyhood of locking your rear brakes before your fronts.

So to sum it up, yes you could theoretically adjust the fronts but since you always want full pressure applied to them and less pressure to the rears, there's no practical benefit to adjusting the front brake pressure. (Excepting of course ABS but that's a whole different can of worms and well beyond the capability of a driver to determine which is why it's handled by sensors and a computer.) However good a feel you have, there's simply no way you can legitimately make the wheel speed calculations that antilock can.

Hopefully this will help clarify the bias valve's function for you or at least present a good analogy or two. Just don't mess with it unless you're positive you know what you're doing as it can have some safety related issues arise if set improperly. Just remember to treat anything suspension, brake, steering, safety, and wheel/tire related on your car with the respect you would if you were working on an aircraft or anything else that someone's life might depend upon not to fail. If you want to dick around with your engine and it blows up, it probably only hurts you and your pocketbook, if it's one of the above categories it might get you or someone else killed and nobody likes that outcome.

And now for a nice anecdotal tangent keep reading, if you don't care, feel free to stop here and you're not missing anything directly related to the subject at hand.

Not to say antilock is infallible either, I had a 2000 Chevy pickup I drove breifly as a work truck a few years ago. That had absolutely the worst ABS programming I've ever experienced. If you were slowing down and hit a bump or pothole, the ABS would interpret that as lockup on that wheel and let up on the braking to it. More than a few times slowing to make a right turn into a driveway I damn near ran into someone because the computer thought my front right had locked up. I eventually popped the fuse killing the ABS in favor of at least having predictability. With ABS off, I never could lock the brakes on that truck anyway with the big metal box it had on the back of it but it did still have plenty of brakes overall so I wasn't too worried about it.

RKD in OKC

There is also a higher 55 barr rear bias brake regulator. Just in case the 33 barr doesn't scare you enough braking in a turn like and on or off ramp and catch up with that slow guy.

I first installed a 33 barr regulator in my 944 turbo to try to get some of the heat out of the front brakes. After putting it in I realized it realized it made the car more agile on turn-in. With a little practice the car would rotate into a nice 4 wheel drift slowly easing off the brakes just as I began turning in.

A word of warning, even under heavy straight line braking you have to stay on top of the steering or the rear will come around.

zerMATT951

Are there any options between the 18 and the 33? Maybe a 25? I've done things like this to other vehicles and I love they way they feel - but they have never been at the point where I'm afraid that the rear's going to pass me...

Is there a happy medium?